Detailed knowledge of the groundwater flow regime is a pre-condition for understanding the geochemical processes in the subsurface. Identification of recharge and discharge areas and mechanisms, as well as flow lines, are essential for delineating trends in geochemical evolution?including changes in As concentrations, and to quantify reaction rates. The Hydrogeology Support Core will provide the tools and expertise for collection and analysis of a broad range of hydrogeological data for Projects 5, 6 and 7, and the Research Translation Core. The following types of data will be obtained and interpreted: 1) longitude, latitude, and elevation;2) stratigraphy of the aquifer from core samples and drill cuttings, hydraulic head, conductivity, storativity, and porosity data;3) geophysical data, primarily frequency EM conductivity, and down-hole logs of resistivity, gamma, flow velocity, and televiewer images;and 4) tracer data (SF6, CH4, Br, 3H/3He, 13C, 14C, 13C, 18O, 2H). The equipment to be provided and operated by the Core include: 1) Multilevel wells, 2) a Geoprobe and other push coring and sampling devices, 3) in situ groundwater monitoring devices, 4) a frequency EM conductivity system, 5) various down-hole logging tools, 6) sample preparation systems for isotopic analyses, 7) gas chromatographs, and 8) noble gas mass spectrometers. Carbon (13C, 14C) and water isotope (18O,2H) analyses will be performed by commercial laboratories. The Core will provide support for the various push/pull and forced gradient in situ experiments, with a focus on characterizing the zone of injections and tracking injected fluids with tracers. Tracer data will also be used to derive groundwater residence times for a range of time scales (from months to 10,000s of years), to identify recharge and discharge areas and mechanisms and to track groundwater mixing in the aquifers. The research support core will provide three-dimensional modeling capabilities for groundwater flow and reactive transport simulations. We will use hydraulic and tracer data as calibration targets for the model groundwater flow and transport model, which will then provide the baseline for reactive transport modeling aided with biogeochemical data obtained by the Biogeochemistry Core.